1. Field of the Invention
The present invention relates to improvements in methods for starting and operating electric motors such as hard disk drive motors, and to improved phase segmented stator winding assemblies with passive switching of phase segments.
2. Description of the Prior Art
In some applications where electric motors are used, the voltage and current as well as the power supply capacity available for motor operation are limited. One example of such an application is a spindle motor for a hard disk drive assembly of a portable computer having power supplied by batteries. In this type of environment, it is important to assure that power is employed efficiently in order to minimize battery drain, and to operate the motor without exceeding a designed maximum voltage and current.
Conflicting demands are placed on the motor during starting and during rotation at the designed operating speed. Before a disk drive assembly is started, the transducer heads typically rest on the surfaces of the disks. When the drive assembly is started, the motor must rotate the disks at an increasing speed until the speed of the disks relative to the heads causes the heads to fly on air bearings above the disk surfaces. A high start torque requirement results from friction between the heads and disk surfaces, and the starting torque problem is compounded by the large number of disks in present day drive assemblies. The high torque requirement presents difficulties, particularly in small form factor drives with small spindle motors.
It is not practical to simply increase the current available to the motor at start up because of the small maximum current available and because of power supply considerations. Another way to increase the starting torque supplied by a motor would be to increase the number of stator windings without increasing the motor drive current. But this approach causes problems at the motor operating speed. If the number of windings is increased, the motor torque constant is increased and the back EMF is larger. This results in decreased motor control voltage available during normal operation. Because the supply voltage is limited, it is desirable to limit back EMF at operating speed to maintain a control voltage differential sufficient for motor speed control.
In the past, segmented phase windings or polyphase windings have been used to permit switching of phase windings of electric motors for various purposes. U.S. Pat. Nos. 1,899,858 and 1,899,859 disclose alternating current motors with polyphase primary windings. Each primary winding includes a plurality of circuits normally connected in parallel with one another. In order to increase the impedance and reduce current when the motor is started, fewer than all the circuits are initially connected to the supply voltage. The remaining circuits are not energized until motor speed increases. Relays are used to control the connection of the winding circuits to the supply voltage.
U.S. Pat. No. 1,992,050 discloses a method and apparatus for starting alternating current motors in which the stator phases are divided into a plurality of independent portions. When the motor is started, all of the phase portions are connected in series. As motor speed increases, one or more phase portions is disconnected and remains idle while the other phase portions remain connected in series. When motor speed increases further, the phase portions are connected in parallel.
Arrangements such as disclosed in the above identified patents are not suitable for computer disk drives and similar applications. One serious defect in those arrangements is that they leave parts of the stator windings inactive under some conditions. Motor efficiency suffers when parts of the windings are not used, and as a result limited power supply capacity such as available in a battery powered system is not used. efficiently. In addition it would be desirable to avoid the use of relays, such as employed in the arrangements of the above patents, and to avoid the use of other active switching elements.